1. Field of the Invention
The present invention relates to a small, wide-angle objective lens and an endoscope apparatus equipped with the objective lens.
2. Description of the Related Art
Since endoscopes are used to conduct observations, for example, in body cavities, there is need for small wide-angle objective lenses for endoscopes. Many of objective lenses for endoscopes proposed conventionally are those with an angle of view (2ω) of about 90° to 140°. For example, Japanese Patent Application Laid-Open Publication No. 10-20189 proposes an objective lens with an angle of view of 112° to 140°. Regarding wider-angle objective lenses, for example, Japanese Patent Application Laid-Open Publication No. 2006-251272 proposes an objective lens with an angle of view of 150° to 170°.
On the other hand, wide-angle objective lenses used in fields other than the endoscope field include, for example, an objective lens described in Japanese Utility Model Registration No. 3140304. The objective lens described in Japanese Utility-Model Registration No. 3140304 is an objective lens for an automotive recording apparatus and has an angle of view of 110° to 175°. Other examples of the wide-angle objective lenses used in fields other than the endoscope field include, for example, objective lenses described in Japanese Patent Application Laid-Open Publication No. 2004-258515 and Japanese Patent Application Laid-Open Publication No. 2004-29282. The objective lens described in the former one, i.e., in Japanese Patent Application Laid-Open Publication No. 2004-258515 is a fish-eye lens for a digital single-lens reflex camera with an angle of view of 180°. The objective lens described in the latter one, i.e., in Japanese Patent Application Laid-Open Publication No. 2004-29282 is a wide-angle lens for a surveillance camera with an angle of view of 128° to 139°.
Furthermore, examples of simple-structured lenses used in fields other than the endoscope field include a lens described in Japanese Patent Application Laid-Open Publication No. 9-297264. The lens described in Japanese Patent Application Laid-Open Publication No. 9-297264 is intended for use in a digital camera and has an angle of view of about 60°.
Recently, there has been demand for a still wider-angle objective lens for use in endoscopes. A reason for the demand for wider angles is related to reduction of inspection time. For example, when observing an object, such as the large intestine, which is a lumen containing folds therein, a user needs to bend a distal end of the endoscope to observe the back side of the folds. In so doing, an objective lens with a wide angle of view makes it possible to observe a large area at once, eliminating the need to bend the distal end of the endoscope or requiring only a small amount of bending, and thereby contributing to reduction of inspection time.
The objective lens according to example 3 of Japanese Utility Model Registration No. 3140304 described above has an angle of view of 175°, which is wider than the angle of view of the objective lens described in Japanese Patent Application Laid-Open Publication No. 10-20189. However, there is demand for a still wider angle for use in endoscopes.